BY CHAS. T. MUSSON. . 887 



It can be said, also, that those species possessing special facili- 

 ties for distribution ; namely, in being on the whole . plentiful, 

 and inhabiting such situations (cellars, gardens, proximity to sea, 

 grass fields, &c.), as would put them in the way of having chances 

 of emigration forced vipon them, have been the fortunate ones, if 

 we may so term it. 



Certain other fresh-water forms one might also expect to see 

 that do not exist here. It would not be surprising to find that 

 European forms of Ancylus, Pisidium, Sphcerhtm, or Planorbis 

 are identical with some of our so-called indigenous forms ; or may 

 find their way here in time. 



Doubtless some forms are better fitted by constitution or other- 

 wise for emigration, and the better able to adapt themselves to 

 circumstances, temperate forms being particularly favoured in 

 this respect. 



We should pai'ticularly notice the fact that our N.S.W. form of 

 H. aspersa, so far as my observation goes (and Mr. Hedley confirms 

 it), is smaller and thinner than the type, and may be considered 

 the variety tenuior, whilst var. conoidea is often seen in New 

 Zealand. The former small, very thin, transparent, reddish, often 

 without bands ; the latter thin, small, and conical. It is a very 

 variable shell. The exceptional thinness of our form might be 

 set down to want of lime in the soil, scarcity of, or ditTerence in, 

 food. This does not, however, necessarily always appear to be 

 the case : on sandy soils on New Red Sandstone in the Midland 

 Counties of England this species attains normal size and thickness. 

 Mr. Carson informs us that shells found in the Melbourne 

 botanical gardens are much larger and thicker than our Sydney 

 form. Of course the curious point is whether the variety tenuior 

 has been introduced here, or whether it has arisen by reason of 

 changed circumstances, from the introduced typical form. 



May it not be probable that the change to our warm and 

 equitable climate is one cause, enabling the animal to do with a 

 thinner protective shell than in colder England? (In S. Europe 

 there is an attenuated form, H. aperta, allied to its larger congener.) 



